(a) Technical Field
The present disclosure relates to a mild hybrid system and a method for controlling the same, which can improvement fuel efficiency.
(b) Background Art
Demands for environment-friendly vehicles has been increased One of the attempts to meet the demands is to provide a mild hybrid system. In general, the mild hybrid system includes an engine and a motor-alternator connected to the engine. The motor-alternator, as a starter motor, receives drive power through an inverter to assist the power of the engine. On the other hand, the motor-alternator, as a generator, generates electricity using energy generated during braking to supply the electricity to a vehicle electrical load.
FIG. 1 is a schematic diagram showing an existing mild hybrid system, and FIGS. 2 and 3 are schematic diagrams showing power transmission paths of the mild hybrid system.
As shown in FIG. 1, the existing mild hybrid system includes a motor-alternator 1, an inverter 7 for controlling the motor-alternator 1, a supercapacitor 2 as an electrical energy storage, and a DC/DC converter 3 for connecting a high voltage of about 30 V and a vehicle electrical load 5 (about 14 V).
Typically, the mild hybrid system has an Idle Stop & Go (hereinafter referred to as “ISG”) function to allow the engine automatically to be turned off when the vehicle stops for a long time and to be turned on when the vehicle is then started. During idle stop, as shown in FIG. 2(a), when the charge voltage of the supercapacitor 2 is greater than the vehicle electrical load 5, the voltage charged in the supercapacitor 2 is supplied to the vehicle electrical load 5 through the DC/DC converter 3. Meanwhile, as shown in FIG. 2(b), when the voltage of the supercapacitor 2 is smaller than a reference voltage (required to drive the vehicle electrical load 5), it is impossible to transmit the electrical energy from the supercapacitor 2 to the vehicle electrical load 5 because the DC/DC converter operates as a buck converter, and thus the electricity of a battery 4 is supplied to the vehicle electrical load 5.
Moreover, as shown in FIG. 3(a), during vehicle braking, the motor-alternator 1 converts the rotational kinetic energy of the wheels into electrical energy and stores the electrical energy in the supercapacitor 2, and the electrical energy stored in the supercapacitor 2 is supplied to the vehicle electrical load 5 during the normal operation of the vehicle such as during cruise or acceleration, which leads to a reduction in the generation load of the motor-alternator 1, thereby improving the fuel efficiency. Further, as shown in FIG. 3(b), during vehicle acceleration, the supercapacitor 2 operates the motor-alternator 1 with the electrical energy recovered during regenerative braking to assist the engine torque, which results in improvement in fuel efficiency, and supplies the reference voltage to the vehicle electrical load 5 to be operated.
However, the existing mild hybrid system has the drawback that the amount of electrical energy recovered during regenerative braking is limited since the capacity of the motor-alternator 1 is small. Therefore, when the vehicle electrical load such as an air conditioner, blower, wiper, audio, etc. is increased above a certain level, it is impossible to cope with the electrical load generated during the running of the vehicle with the electrical energy recovered during regenerative braking, which leads to a reduction in fuel efficiency. More particularly, as shown in FIG. 4, which shows the change of improvement in fuel efficiency according to the electrical load in the existing mild hybrid system, when the vehicle electrical load is increased, the improvement in fuel efficiency obtained by a reduction in the generation load of the motor-alternator 1 is sharply reduced. Moreover, the DC/DC converter 3 should be designed to provide a high capacity (more than 1.5 kW) so as to cope with the capacity of the entire electrical load, which leads to an increase in the cost and an inefficiency of the system.
In addition, the existing mild hybrid system starts the vehicle using the electrical energy stored in the supercapacitor 2. However, since the level of electrical energy stored in the supercapacitor 2 is continuously reduced by self-discharge in terms of the characteristics of the supercapacitor 2, the level of electrical energy is reduced to a level at which the vehicle cannot be started when it is left for a long time. As a result, the vehicle should be started either by using the existing starter system or by operating the DC/DC converter 3 in a reverse direction to charge the supercapacitor 2 with the electrical energy of the 12 V battery 4. Namely, the existing mild hybrid system has the drawback that the DC/DC converter 3 should be configured as bidirectional or the existing starter system cannot be eliminated.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.